The invention relates to a method and a circuit arrangement by means of which a stepper motor can be operated by an adaptive control across a broad rotational speed range including a standstill in which the motor is electrically fixed in a specific rotational position, and with high precision corresponding to a predefined motor current course.
It is generally known that in stepper motors a magnetic rotor is turned stepwise by each a small angle by means of a controlled rotating electromagnetic field which is generated with static motor coils.
Frequently, it is desired to rotate the motor with as far as possible small step angles, in order to achieve an as high as possible resolution and exactness of the positioning and a uniform course of the motor torque. For these reasons, instead of the known full-step and half-step operation, the so called micro-step operation is preferred in which the currents flowing through the motor coils are not only switched on and off, but increase and decrease in a certain manner. The resolution and the uniformity with which the stepper motor conducts the micro-steps is in this case substantially dependent on the number of different current amplitude values with which the motor coils can be operated and how exactly these can be kept. Usually, it is most appropriate to excite the motor coils with a sinusoidal- and cosine-wave, respectively, because with this a very continuous and jerk-free rotation of a microstep-optimized motor and by this a calm motor operation can be obtained.
For electrically controlling stepper motors, especially in the micro-step operation, known chopper methods are used, with which by means of a motor-supply voltage (direct voltage) for each instant of time by means of current pulses the current direction, current value and current course is impressed into each of the motor coils, which are given by a specified current (target coil current), in order to drive the motor by the thus induced rotating magnetic field.
In this case it is usual to measure the actual current flowing through the motor coils and to regulate it in dependence thereon in positive and negative direction and polarity, respectively, by means of appropriately activated and temporally dimensioned chopper phases (ON, SD, FD) of a chopper method such that the motor current at least substantially coincides in each chopper phase and by this over the entire course with the course and the polarity of the related target coil current. This operation shall be denoted in the following as a current-regulated operating mode.
In such a chopper method, usually three different chopper phases (coil current phases) are distinguished, namely ON-, FD- and SD-phases.
During the ON-phase (also called positive switch-on phase) the coil current in a coil is actively driven into the coil in the direction of the instantaneously specified polarity and direction, respectively, of the coil current, so that the amount of the coil current increases relatively quick and continuously (switch-on period) until it has reached its instantaneous target value and the ON-phase is then terminated. The direction of the coil current which is impressed by such an ON-phase is thus equal to the instantaneous polarity and direction, respectively, of the coil current.
The polarity of the coil current is in case of a sine-shaped coil current for example positive in the first and second quadrant and negative in the third and fourth quadrant.
During the FD-phase (negative switch-on phase) the coil current is actively reduced against the just specified polarity of the coil current by reversing the poles of the coil and feeding back the coil current into the current supply until it has reached its instantaneous target value and the FD-phase is then terminated. Alternatively, the FD-phase can also be terminated without regulation after the expiration of a pre-set time duration such that due to experience in a certain application during the related FD-phase the maximum necessary decrease of the coil current is reached without actually measuring the same. In any case, the FD-phase is provided to reduce the coil current particularly during the time of decreasing amounts of the coil current (i.e. during the second and third quadrant of a sine-shaped coil current) relatively quickly.
The third chopper phase is the recirculation phase or SD-phase, in which the related coil is not actively controlled but rather short-circuited or bridged, so that the coil current, due to the inner resistance of the coil and the counter-EMF, decreases only gradually (i.e. slower than during the FD-phase). During this phase the coil current can usually not be measured, so that the SD-phase has to be terminated after the expiration of a pre-set time duration, wherein usually for all SD-phases the same constant time duration is pre-set.
These three chopper phases are therefore temporally activated, combined and dimensioned by means of chopper switch signals, generated by a chopper and supplied to a motor coil driver circuit, such that the actual coil current follows over its entire (e.g. sine-shaped) course, namely during the increasing and decreasing sections of the coil current, as far as possible promptly and exactly the corresponding specified current (target coil current) for the related motor coil and is at least substantially not influenced by the voltage which is counter-induced by the rotor within the motor coils (counter EMF) or other effects. In other words, each period of the actual coil current is composed of a plurality of chopper phases, by means of which each target coil current value of the current period at each instant of time of the activation of the related chopper phase is impressed into the coil.
However, it has revealed that during this current-regulated operating mode particularly in case of a low rotational speed and standstill of the motor at an electrically fixed position (i.e. in a certain position of rotation) short-time current variations due to fluctuations of the regulation may occur in the audible frequency range which is undesired. Such fluctuations of the regulation result from measuring or sampling noises, couplings within the motor and from interferences from other circuits or from the supply voltage.
Furthermore, it can be difficult at low motor currents in connection with the resulting only very short duration of the ON- and (if any) FD-phases and due to transient effects and blank times, to reliably measure during these short times the actually flowing coil current and to compare it with the instantaneous target coil current value. The phases are therefore usually extended to a certain minimum value.
It is desirable to provide a method and a circuit assembly for operating a stepper motor, with which with a relatively small circuit complexity an optimized (and particularly calm) operation of a stepper motor, particularly with respect to a desired or target coil current course, can be obtained over a broad rotational speed range, i.e. between a standstill of the motor in which the motor is electrically fixed in a specific rotational position and a motor-related highest rotational speed.
An aspect of the invention is preferably applied in micro-step operation, however, it can be applied in full-step and half-step operation as well.
The dependent claims disclose advantageous embodiments of the invention.